In electromechanical protective devices, in particular in circuit breakers, bimetal or trimetal strips are used as overload releases. In order to achieve the desired release characteristics, the metal strips generally have either a heating winding or a heating pack.
Heating windings are metal wires or bands that are wound around the bimetal strip. Between the bimetal strip and the heating winding there is an electrical insulator, for example a glass-fiber fabric, in order to prevent a short-circuit of the individual heating conductor windings to the bimetal strip. At the upper end of the bimetal strip, the heating conductor and the bimetal are welded to one another.
Heating packs comprise a metal band that is folded into the form of a pack and fastened to the bimetal strip. Between the individual folds there is an electrical insulator, such as for example mica flakes, so that current flows through the metal band of the heating pack in its entire length. In the case of bimetal or trimetal strips with a heating winding, current flows through the bimetal. By contrast, in the case of bimetal strips with a heating pack, generally no current flows through the bimetal. The heating pack with current flowing through it only heats the bimetal indirectly. Therefore, no feeding between the heating pack and the bimetal is required here.
The production costs for wound bimetal strips are lower than for packeted bimetal strips. The reason for this is the easier automatability, the lower material requirement and the easier integration, from a technical installation and welding viewpoint, of the wound bimetal strip in the current path of the associated device. For this reason, it is endeavored to use wound bimetal strips with preference.
However, the use of wound bimetal strips especially reaches its limits in the weldability of the required bimetal and heating conductor materials. Here, the resistance forge welding method is used. While those materials that are used in protective devices of low setting ranges can be welded relatively well, the weldability of materials that are required for high setting ranges decreases greatly. Therefore, wound bimetal strips are found predominantly in devices with low setting ranges. Packeted bimetal strips are used in devices with higher setting ranges.
If it were technically possible for welding also to be used for connecting the materials that are required in the case of bimetal strips for high setting ranges, wound bimetal strips could take the place of many types of packeted bimetal strip. This would achieve a reduction in the production costs.
The problems that arise in the resistance forge welding of materials that are required in bimetal strips for high setting ranges are often avoided by using packeted bimetal strips. In that case, however, the higher production costs have to be accepted. Furthermore, it is attempted to push the limit of the setting range for which wound bimetal strips can still be used up as far as possible by refining the resistance forge welding method. Measures in this direction are, for example, that of replacing the alternating-current technique for the welding current sources by a direct current technique (inverter) that can be regulated with better control, that of using better materials for the welding electrode or controlled welding tongs with a servo drive (motor head) and integrated monitoring of the welding force, moving-on movement and moving-on displacement. Further measures are that of reworking welding spatter that occurs, increased inspection of the as-welded strength of samples from destructive testing or else the acceptance of low electrode lifetimes (for example 200 welding operations). In addition, the heating windings are often dimensioned such that a welded area of 50% of the heating conductor cross section is sufficient to carry the current. This causes greater use of material, and consequently higher costs. It also has to be taken into consideration that, in the case of material combinations that are difficult to weld, welded areas of 100% of the current-carrying heating conductor cross section cannot be produced in practice by the resistance forge welding method.